1. Field of the Invention
The present invention relates to a toner for use in electrophotography. Particularly, the present invention also relates to an image forming method and a process cartridge using the toner.
2. Discussion of the Related Art
An electric or a magnetic latent image is generally developed with a toner to become visible. The toner typically comprises colored particles in which a colorant, a charge controlling agent, and other additives are contained in a resin. Toner manufacturing methods are broadly classified into pulverization methods and polymerization methods. The pulverization method includes steps of melt-mixing toner components, such as a colorant, a charge controlling agent, and an offset inhibitor, with a thermoplastic resin so that the toner components are uniformly dispersed in the resin; pulverizing the melt-mixed mixture; and classifying the pulverized mixture.
The pulverization method is capable of providing a toner having desired toner properties to some extent. Cross sections made by the pulverization typically include cracks. When a stress is externally applied to the cracks, ultrafine particles tend to peel off therefrom. In a two-component development process, ultrafine particles tend to be produced from the cross sections (i.e., the surface of the toner particle) and adhere to the surface of a magnetic carrier, due to the application of agitation stress thereto. Thereby, the charging ability of the carrier deteriorates and the toner cannot be charged to the desired level.
In attempting to solve the above problems of the pulverization method, unexamined published Japanese Patent Application No. (hereinafter referred to as JP-A) 09-43909 discloses a suspension polymerization method as a toner manufacturing method. The suspension polymerization method is capable of providing a toner not only including few cracks, but also having a spherical shape and a narrow particle diameter distribution. The use of the spherical toner is capable of improving latent image reproducibility, resulting in producing high quality images. However, such a spherical toner is hardly charged, because the spherical toner tends to slip when triboelectrically-charged by a carrier in a two-component development process. In particular, in a development process in which fresh toner particles are successively supplied, such as a continuous printing of a high-image-proportion image, the fresh toner particles cannot be rapidly charged. Therefore, background fouling in that the background portion of an image is soiled with toner particles tends to be caused.
There is another disadvantage that spherical toner particles are difficult to remove with a cleaning blade when remaining on a photoreceptor. When an image having a low image area proportion is developed or transferred, few toner particles tend to remain on the photoreceptor, which are easily removed. In contrast, when an image having a high image area proportion (such as a photograph) is developed or transferred or paper is not efficiently supplied, toner particles which are not transferred and remain on the photoreceptor tend to cause the background fouling. Such remaining toner particles also tend to contaminate a charging roller, configured to contact-charge the photoreceptor, and deteriorate the charging ability thereof.
In attempting to solve the above problems, JP-As 08-62893 and 2007-79223 have disclosed toners in which a polymerization toner and a pulverization toner are mixed. The pulverization toner is mixed as an auxiliary component so that the resultant toner is easily removed with a blade. However, the pulverization toner, which includes cracks, cannot be prevented from producing ultrafine particles and tends to adhere to the carrier. As a result, charging ability of the carrier deteriorates. On the other hand, the polymerization toner, which is a main component of the resultant toner, tends to slip on the surface of the carrier when supplied to a development device. Therefore, the polymerization toner cannot be sufficiently frictionized and cannot be rapidly charged, resulting in causing background fouling. These problems cannot be solved even if the mixing ratio of the polymerization and pulverization toners is varied.